You are right, it is a bit incoherent.
But what would the critics be if we really walked "down" the alphabet?
In German you don't say "down the alphabet" when you go from A to Z.
The alphabet is an index like if we would use numbers. It would be odd to count them up by the down-key.
Unfortunately there are less industry standards visible today, than it used to be. (See the Audi example).
It is often about "moving a cursor over the content" or "moving the content in the window", witch is an opposite action. (Apple example).
At lease I haven't seen up/down buttons yet in a vertical alignment where the lower button increases the index. Have you?